Data Formats

A file with .AMR extension is a data format originally developed by Ericsson. It is used by many 3G cell phones for voice recordings such as MMS messages and also live voice telephony compression. It incorporates the Algebraic Code Excited Linear Prediction (ACELP) algorithm, which is designed to efficiently compress human speech audio. This speech encoding format algorithm is mostly used during live GSM telephony to adapt to the channel bandwidth and noise. The AMR ACELP coder will lower the compression rate as cellular interference increases, thus enabling more error correction to be applied. AMR was adopted as the standard speech codec by 3GPP in October 1998.

The ACELP algorithm uses link adaptation to select from one of eight different bit rates based on link conditions (interference and channel noise). That is why it is called multi-rate. It is adaptive because if the channel is noisy, thus presenting a significant percentage of loss of data and resulting in cut-off voices and hiccups during conversation, thus when the algorithm detects that the channel cannot support a certain amount of audio quality, it will automatically reduce the quality of the voice encoding. To the human ear the difference in quality is negligible, but this enables a person to talk smoothly over a period of time without hiccups in transmission. The ACELP algorithm is also used to harmonize the codec standards amongst different types of cellular systems.

Many modern mobile telephone handsets will allow you to store short recordings in the AMR file extension. There exist both Open Source and commercial programs to convert between AMR and other formats such as MP3, although it should be noted that AMR encoding is a speech format and is unlikely to give ideal results for other types of audio.

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